This invention relates to a process for the production of phase-pure highly reactive lead metal niobates of perovskite structure corresponding to the general formula Pb.sub.3 Me(II)Nb.sub.2 O.sub.9 or Pb.sub.2 Me(III)NbO.sub.6, where Me.dbd.Mg, Fe, Co, Ni, Cr, Mn, Cd, Cu and/or Zn, in which perovskite intermediates are formed from corresponding salt solutions, separated off, dried and calcined at temperatures of 500.degree. to 1000.degree. C.
By virtue of their high dielectric constant and their high electrostrictive coefficient, ferroelectric, such as for example PbMg niobate, PbNi niobate or PbZn niobate, are playing an increasingly important role. Compounds such as these with their perovskite structure correspond to the general formula Pb.sub.3 Me(II)Nb.sub.2 O.sub.9 or Pb.sub.2 Me(III)NbO.sub.6, where Me.dbd.Mg, Fe, Co, Ni, Cr, Mn, Cd, Cu, Zn (Appl. Phys. Lett., 10(5) 163-165 (1967)).
There are several known processes for the production of complex ferroelectric perovskites with a transition metal belonging to the 5th secondary group:
According to J. Am. Ceram. Soc. 71 (5), C-250-C-251 (1988), the oxides are mixed and subsequently subjected to a solid-state reaction at very high calcination temperatures. It is extremely difficult to produce phase-pure perovskites containing more than 95% perovskite phase by these ceramic processes. A stable pyrochlore phase unavoidably occurs during this solid-state reaction. It is known from J. Am. Ceram. Soc. 67 (5) 311-314 (1984) that Nb.sub.2 O.sub.5, for example, can be reacted with MgO in a preliminary solid-state reaction, a substantially phase-pure perovskite being obtained in the subsequent reaction with PbO in accordance with the following formulae: ##STR1##
However, in view of the high temperatures of 1000.degree. C. in the preliminary reaction to columbite, an only moderately reactive Mg niobate is obtained from the mixed oxides mentioned, so that the subsequent reaction with PbO can only be carried out successfully at relatively high temperatures. In the final production stage, the perovskite can only be sintered at temperatures of around 1200.degree. C.
Wet chemical processes have proved to be of greater advantage for the production of complex perovskites. Thus, J. Am. Ceram. Soc. 72 (8), 1333-1337 (1989) describes the hydrolysis of an alkoxide mixture while Ep-A 294 991 describes co-precipitation from an alcoholic oxalic acid solution. The calcination of a gel produced from NbCl.sub.5 or Nb(OR).sub.5 and metal salts by addition of H.sub.2 O.sub.2 and citric acid is known from Advances in Ceramics, Vol. 21, 91-98 (1987). The co-precipitation method generally gives highly reactive intermediate products which react even at low temperatures to form the corresponding phase-pure perovskites. The sintering properties of these very fine powders are correspondingly good. However, these wet chemical processes have hitherto been extremely uneconomical. Thus, the alkoxide method has the disadvantage that the starting materials are difficult to produce and handle.
In the oxalate method, it is not possible to co-precipitate all the components because problems arise through the considerable differences in the solubility of the metal oxalates in an alcohol/water mixture. Nevertheless, a very fine-particle powder having good sintering properties is obtained even in this method. The most serious disadvantage of the oxalate method lies in the very large quantities of alcohol required for quantitatively precipitating all the components. The poor solubility of niobium oxalate is another disadvantage. The known processes mentioned in the foregoing either have the disadvantages that the calcination and sintering temperatures are very high, as in the ceramic processes, or that significant complications and high costs are involved, as is the case with the wet chemical processes. The same also applies to the described citrate process. The known starting materials for this process are corresponding metal salts and niobium chloride or niobium alkoxide. It is only be addition of hydrogen peroxide (to form a water-soluble niobium peroxo complex) and citric acid that an aqueous gel can be produced from all the components together. In this process, however, co-precipitation is not possible because the metal complexes formed (metal/citrate complex and niobium/peroxocitrate complex) are extremely stable. Accordingly, the complexes do not decompose, even in the presence of excess ammonia. Some of the water-soluble Me citrate complexes are so stable (as, for example, where Me.dbd.Pb, Mg, Fe, Ni, Co, Mn, Nb, Ta, Zn, Cd citrate) that, when ammonia is added, the corresponding hydroxide does not precipitate. Binary mixtures of the metal citrates (Pb-Nb, Me-Nb) behave in exactly the same way as the individual components.
Accordingly, the principal object of the present invention is to provide a process for the production of perovskites which does not have the disadvantages of the prior art.